1. Field of the Invention
The invention relates to a detector for detecting electrically neutral particles, to a converter device for a detector for detecting electrically neutral particles, to a method for producing a converter device and to a detection method for detecting electrically neutral particles.
2. Description of the Related Art
The use of low-energy neutron radiation, known as thermal and cold neutrons, is an important method used in science (for example for applications in physics, chemistry, biology and medicine) and engineering (for example non-destructive testing). The basis for all the application areas in science and engineering is the detection of these neutrons, and consequently detectors and methods for detecting neutrons have become economically very important in recent decades. For physical reasons, the detection of neutrons can only be achieved as a result of a nuclear reaction thereof with a neutron converter. This causes the neutrons to be trapped or absorbed by the atomic nuclei of the converter, with these nuclei then spontaneously breaking down. The most high-energy, electrically charged fragments formed during this breakdown are jointly referred to as conversion products and can then be detected on account of their ionizing effect.
Hitherto, the gas helium-3, the atomic nuclei of which comprise two protons and one neutron, has predominantly been used to detect neutrons. In what are known as gas detectors, this helium isotope is added to the actual counting gas of the detector in predetermined quantities. Neutrons which are to be detected are absorbed by the helium-3 nuclei, which subsequently spontaneously break down in accordance with the following nuclear reaction 3He+1n→3H+1p+764 keV, the tritium nucleus containing a quarter and the proton three quarters of the reaction energy. These conversion products, as high-energy, charged particles, have an ionizing effect on the counting gas of a gas detector of this type. When helium-3 gas detectors are being used to detect neutrons, the conversion products in the counting gas therefore generate charged particles, in particular free electrons. These primary electrons are guided to the electrodes of a readout structure as a result of the application of an electrical drift field. Suitable shaping of the readout structure means that the electrical field in the vicinity of the electrodes is so high that the primary charge can be hugely amplified with the aid of secondary gas ionization processes (gas amplification). The total charge generated in this way is subsequently collected at the electrodes and is fed to an electronic evaluation device via a preamplifier.
However, neutron detectors of this type in the form of conventional gas detectors with helium-3 as the neutron converter have considerable drawbacks. Specifically, to achieve an attractive detection efficiency of, for example, approximately 50% for thermal neutrons and at the same time to be able to determine the location of incidence of the neutrons, with a gaseous neutron converter such as helium-3, detectors of this type have to be operated at a gas pressure of 5 to 10 bar. The high operating pressure means that this requires complex and expensive pressure vessels. On account of the design limitations of the pressure vessels, detection of neutrons above large detection areas can only be achieved with the aid of large detector arrangements which are in the form of a matrix and comprise a multiplicity of small individual detectors. For example, the IN5 neutron spectrometer produced by Laue-Langevin in Grenoble for angularly resolved neutron detection has 1400 individual helium-3 neutron detectors (cf. “The yellowbook guide to neutron research facilities at ILL”, Institute Laue-Langevin, Grenoble, December 1997). The spatial resolution of approx. 2 cm×10 cm and the typical counting rate acceptance of 10,000neutrons detected per second and cm2 of a neutron detector of this type are, however, highly unsatisfactory.
Although the poor resolution and the low counting rate acceptance can be improved by combining helium-3 as converter with a microstrip detector (MSGC) to approx. 2 mm×2 mm and one million neutrons per second and cm2 (cf. Vellettaz et al., “Two-dimensional gaseous microstrip detector for thermal neutrons”, Nuclear Instruments and Methods A 392 (1997), pages 73 to 79), the structure of these detectors is very complex and expensive even for a detector area of only 100 mm×100 mm, on account of the high gas pressure. Furthermore, the MSGC technology has proven to be highly susceptible to faults. A further drawback is the poor time resolution of the gas detectors which have been described to date. Since thermal neutrons are absorbed anywhere in the depth of the gas volume, the location of the neutron absorption and therefore the passage time through the detector are not accurately known. The inaccuracy in the passage time and therefore in the time resolution is approximately 10 μs for thermal neutrons at typical depths of the absorbing gas volume of approximately 2 cm.
Furthermore, neutron scintillation detectors are known for the detection of neutrons. With detectors of this type, a solid neutron converter is admixed with a solid or liquid scintillator, for example in the form of a fine powder (cf. G. B. Spector et al., “Advances in terbium-doped, lithium-loaded scintillator glass development”, Nuclear Instruments and Methods A 326 (1993), pages 526 to 530). The conversion products which are formed in a neutron detection reaction deposit their energy in the scintillator. The light radiated onto them using the scintillator is then detected in a position-sensitive manner using a suitable light detection system. Detectors of this type have typical detection efficiencies of 20% to 40%. However, detection of the scintillation light causes problems. Since detection concepts of this type are relatively highly sensitive to X-radiation and gamma radiation, which cannot be avoided in a reactor or neutron environment, their possible applications are greatly restricted. In particular, this background which is attributable to X-radiation and gamma radiation makes detectors of this type unsuitable for the individual detection of neutrons or the detection of very low neutron intensities, and consequently detector systems of this type are only able to detect distributions with intensive event rates in a positionally dependant manner.
It is an object of the invention to provide a detector for electrically neutral particles, in particular neutrons, which combines a high detection sensitivity with a simple and therefore inexpensive design. A further object of the invention is to provide a converter device for a detector of this type for detecting neutral particles, and a corresponding method for producing the converter device. A final object of the invention is to propose a corresponding method for detecting electrically neutral particles.